Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
BE-A 784,615 discloses a photographic material comprising a paper support, characterized in that the support is coated with a layer having a Sheffield smoothness between 5 and ca. 100, the layer containing a first binder of the type polysaccharide, a pigment, a second binder, a photosensitive compound, including 2,4-dihydroxy-benzophenone, and an oxidation-reduction imaging forming system.
EP-A 713 133 discloses a two sheet thermal imaging system consisting of (i) a donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver and (ii) a receiving element comprising on a support a receiving layer comprising a silver source, capable of being reduced by means of heat in the presence of a reducing agent, a binder and a stabiliser selected from the group consisting of benzotriazoles, heterocyclic mercaptanes, sulphinic acids, 1,3,4-triazo-indinolines, 1,3-dinitroaryl compounds, 1,2,3-triazoles, phthalic acids and phthalic acid derivatives.
EP-A 809 142 discloses a recording material comprising a support, a thermosensitive element and a protective layer therefor, the thermosensitive element comprising a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith and a binder, wherein the thermosensitive element further comprises in reactive association with the substantially light-insensitive organic silver salt and the organic reducing agent a substituted, exclusive of groups having an exclusively electron withdrawing character, or unsubstituted compound with an unsaturated 5-membered heterocyclic ring annulated with an aromatic ring system, the ring consisting of nitrogen and carbon atoms with at least one of the nitrogen atoms having a hydrogen atom and none of the carbon atoms being directly linked to a sulfur atom or being part of a carbonyl-group.
Substantially light-insensitive black and white two sheet thermographic recording materials according to EP 713 133 produce an image in the receiving sheet with imagewise transferred reducing agent and hence totally different stability requirements to monosheet materials. Substantially light-insensitive black and white monosheet thermographic recording materials including non-N-substituted benzotriazoles according to the teaching of EP-A 809 142 exhibit poor stability to direct sunlight.